The field of the invention relates to hydrostatic transmissions. More specifically, the field is that of neutral shifting mechanisms for hydrostatic transmissions.
A hydrostatic transmission transmits energy from mechanical rotary motion to fluid (typically oil) movement, then back to mechanical rotary motion. Within a transmission, a cylinder unit rotates on a pintle, with pistons positioned within the cylinders and attached to slippers mounted in an expander band, so that as the cylinder unit rotates the pistons are driven by the slippers which engage the surrounding eccentric annular track ring. The pistons of the driving cylinder unit create a pressurized fluid flow that drives a receiving set of pistons which rotate an output shaft. The transmission ratio is directly proportional to the eccentricity of the track ring relative to the pintle. U.S. Pat. No. 4,635,535 and 4,691,512, which are incorporated by reference herein, describe hydraulic units which may be used within a hydrostatic transmission.
One common application of a hydrostatic transmission is with a riding lawn mower. Although riding lawn mowers are usually self-propelled, in many situations the use of a riding lawn mower involves manually pushing the mower. For example, to guide the mower over obstacles such as ditches or foliage an operator may not feel comfortable riding the mower. Alternately, vehicles are often rearranged in a garage or storage area when their engines are not running. Also, an operator may be required to manually push the mower because of an engine failure.
Hydraulic transmissions present a special problem when the vehicle is to be moved manually. In gear transmissions, the gears coupling the input and output shafts can be disengaged, so that moving the vehicle manually is possible with little resistance from the transmission system. However, with hydraulic transmissions, the hydraulic motor remains engaged and presents substantial resistance to the movement of the vehicle.
Conventional hydrostatic transmissions achieve a neutral state by releasing the pressure within the fluid flow so the pistons have no driving force. Opening check valves within the pintle of the hydrostatic transmission short circuit the oil passages which connect the hydraulic pump and motor. The short circuit in the oil passages prevents the rotation of either the input or output shafts from effecting the movement of the other. However, using check valves presents difficulties involving unseating the valve which requires complicated and unwieldy control mechanisms within the limited area inside the casing of the transmission, adding to the expense while decreasing the efficiency and reliability of the hydrostatic transmission.
What is needed int he art is an improved neutral shift mechanism which conveniently operates within a hydrostatic transmission. Further, an improved shifting mechanism which provides a locked neutral position in the shift control for a hydrostatic transmission is needed.